1. Field of the Invention
The present invention relates to a heat exchanger assembly, and more particularly, to an evaporator for a heating and/or air conditioning system (HVAC) for automotive vehicles.
2. Description of the Prior Art
An evaporator of the type to which the subject invention pertains exchanges heat between a cooling fluid and air. A stack of virtually identical plates are positioned symmetrically in pairs having mating edges and a concave region delimited by the edges to define a fluid passage. The plates have tubular projections defining an inlet for entering fluid to the passage and an outlet for exiting fluid from the passage to thereby establish a direction of fluid flow. Each inlet is connected to the outlet of the preceding pair of plates and each outlet is connected to the inlet of the next pair of plates. Actually, each pair of plates includes a central rib to define a U-shaped passage having a fluid entering leg and a fluid exiting leg interconnected by an open bottom. Examples of such heat exchangers are described in U.S. Pat. No. 5,111,878 to Kadle and U.S. Pat. No. 5,409,056 to Farry, Jr. et al.
Hot and humid air flows between the consecutive pairs of plates. The plates are usually stamped of thin gauge metal and a plurality of dimples is stamped into the plates to project into the passage to interact with fluid flow through the passage. These dimples can be identical in shape, position and orientation or they can be of various shapes as illustrated in U.S. Pat. No. 6,289,982 to Naji. They project into the interior of the passage formed by the pairs of plates and thus allow better heat exchange by agitating the cooling fluid flow, and especially by promoting its movement in a turbulent flow. These dimples can be formed by an assembly method, particularly by brazing two bosses opposite each other. In this case, the plates forming a pair of plates are the same as one another, and each boss has an equivalent height of approximately one-half of the depth of the U-shaped passage, that is to say of the distance from the opposing plates.
Unfortunately the flow of cooling fluid in this type of evaporator can produce a noise, particularly a “whistling”, i.e., a tonal noise emanating from a plate-type evaporator used in certain automotive climate control systems under transient conditions. It is believed that this tonal noise occurs when gaseous refrigerant at sufficiently high velocities flows over the first dimples. It is further believed that the tonal noise is caused by periodic flow instability (manifested as vortices) in the wake of the first dimples. When the vortex shedding frequency is near the natural frequency of the gas column perpendicular to the direction of flow, a strong acoustic oscillation of the vapor column is excited, and it is this resonant oscillation that is believed to be the source of the tonal noise or whistle.
It is believed that a flow-induced whistle occurs when superheated refrigerant flows through the dimpled tube plate passages. When refrigerant vapor at sufficiently high velocity flows over the dimples in the evaporator tube plates, the flow sets up a periodic flow instability, also known as vortices, in the wake of the dimples. Initially, as the flow velocity increases, the frequency of the vortex shedding also increases. This phenomenon is known as “Strouhal effect”.